media-video/mplayer unsets CFLAGS. This causes some rather nasty breakages on certain systems, see bug #59506 (Required CFLAGS being killed metabug) for details.
Doesn't the ebuild contain why this is the case?
"Upstream doesn't like it" is no excuse for killing ABI flags. By all means strip out optimisation flags and use upstream's instead if they're going to have a hissy fit otherwise. However, ABI-related flags must be kept or the code produced will be unusable.
Hmm, thought I marked this fixed... well anyways, it's fixed and uses custom